The present invention relates to the field of wireless communications systems, and in particular, to a method and system for interference mitigation in adaptive array systems.
One advance in increasing the capacity of communication systems has been in the area of resource sharing or multiple access. Examples of multiple access techniques include code division multiple access (CDMA), frequency division multiple access (FDMA), and time division multiple access (TDMA). For example, in a TDMA system, each remote user terminals communicates with a hub communication device (e.g., a base station) in a frequency channel shared with other remote user terminals, but in its own (i.e., non-overlapping) time slot. As such, in a TDMA system, multiple remote user terminals may communicate with the hub communication device within the same frequency channel, but within non-overlapping time slots. (The term xe2x80x9cchannelxe2x80x9d as used herein refers to any one or a combination of conventional communication channels, such as frequency, time, code channels).
Unfortunately, communications systems, especially those employing multiple access techniques, may suffer from inter-channel interference (inter-channel interference is also sometimes referred to as adjacent channel interference; however, the term inter- channel interference is used herein to emphasize that interference may occur between channels that are not necessarily adjacent, but may nonetheless affect each other). For example, in an FDMA cellular communication systems, when a base station transmits a downlink signal to a first receiver (which may be a cellular telephone handset or other remote user terminal) on a primary frequency channel, a second receiver that is tuned to receive in a non-primary frequency channel, which channel may be adjacent to or relatively near the frequency band of the primary frequency channel, may nonetheless experience inter-channel interference due to transmitter, receiver, and/or channel characteristics or limitations that cause energy from the primary downlink signal to be detected as interference on one or more non-primary channels. Similarly, in a TDMA system, receivers operating in adjacent time slots may experience inter-channel interference. Nonetheless, this is currently employed in some systems, such as GSM system.
Inter-channel (and/or co-channel) interference experienced by receivers, such as remote user terminals, that are not the intended recipient of the xe2x80x9cprimaryxe2x80x9d transmission of a base station or other communication device may be caused by one or a combination of factors attributed to the limitation(s) of the receiver(s), the characteristics of the channel and/or environment, and/or by generation of xe2x80x9cghostxe2x80x9d signals by the transmitter (e.g., by the base station). For example, factors that are attributed to limitations of a receiver, such as a remote user terminal, and which factors may cause inter-channel interference to occur include, but are not limited to, relatively limited dynamic range in the receive path of the remote user terminal, phase noise in the remote user terminal""s oscillator, relatively poor analog and/or digital filtering or channel selectivity of the remote user terminal. On the other hand, factors attributed to a transmitter, such as a base station, may also cause inter-channel (and/or co-channel interference) that may be experienced by one or more receivers. For instance, a transmitter may generate unwanted xe2x80x9cghost signalsxe2x80x9d to appear on xe2x80x9cprimaryxe2x80x9d or xe2x80x9cnon-primaryxe2x80x9d channels when the transmitter transmits a downlink signal on the primary channel.
Unfortunately, techniques for alleviating inter-channel interference by improving the remote user terminal""s selectivityxe2x80x94i.e., its ability to discard unwanted signals in nearby frequency, time, and/or code channelsxe2x80x94generally entail additional cost or power consumption. On the other hand, relatively limited selectivity of a remote user terminal""s receiver may cause a number of undesirable effects in a communications system. In fact, if adjacent channels are occupied by signals of sufficient power, the resultant interference to the remote user""s receiver may render the remote user terminal relatively unreliable or even inoperable.
One technique to reduce or eliminate inter-channel inteference is to leave unoccupied (i.e., unused) adjacent channels and/or other relatively nearby channels that may be susceptible to (or cause) inter-channel interference. For example, if a remote user terminal in communication with a base station is using a given channel, the base station may be programmed not to assign adjacent or other relatively nearby channels to other remote user terminals whose relatively limited channel selectivity may render such adjacent or nearby channels susceptible to inter-channel interference. However, by leaving some otherwise usable channels unused, this solution leads to a relatively significant loss in spectral efficiency. In systems where there may be a relatively large number of remote user terminals, such a loss in spectral efficiency may render this solution impractical.
Another prior technique for reducing inter-channel interference involves dynamic channel allocation. One example of dynamic channel allocation is employed in the Personal Handyphone System (PHS), a cellular network architecture currently implemented in a number of geographical areas, including, for example, in portions of Japan. PHS remote user terminals (also known as PHS handsets) are capable of transmitting control messages to a PHS base station. When a PHS handset detects a deteriorated signal quality (e.g., due to inter-channel interference), the PHS handset informs the PHS base station, via a control message, that a new channel is needed, and such new channel may be allocated by the PHS base station to the PHS handset during a communication session (e.g., during a voice or data xe2x80x9ccallxe2x80x9d).
However, before a PHS handset accepts a newly assigned channel, the handset measures the interference on the newly assigned channel to determine whether it is significant relative to a threshold. When the PHS handset performs the measurement of interference on the newly assigned channel, the handset uses the same receiving apparatus that is used during normal traffic of voice or data exchange with the PHS base station. As such, even during the measurement phase for a newly assigned channel, the PHS handset may experience interference from signals on adjacent or nearby channels. If the level of such interference is too high, for example, as compared with a threshold, the PHS handset may again request a new channel from the base station.
Eventually, if network loadxe2x80x94namely, the number of users (e.g., PHS handsets) or other signal sources or receiversxe2x80x94does not exceed a threshold, the PHS handsets and base stations in the PHS network may find a pattern of time slots and frequencies that facilitate communication with a tolerable amount of inter-channel interference. If, on the other hand, no suitable channel can be found by a PHS handset in a number of attempts or within a predefined time-period, a call may be droppedxe2x80x94i.e., communication may involuntarily be terminated between the PHS handset and the base station. Furthermore, even if communication is not terminated, voice quality or data integrity is typically significantly reduced when a PHS handset switches between channels.
Adaptive arrays (also known as xe2x80x9csmart antennasxe2x80x9d), which employ antenna arrays along with signal processing hardware and/or software, also have been utilized to decrease interference and improve performance in wireless communications. Antenna arrays typically include a number of antennas that are spatially separated and coupled to one or more digital signal processors and/or general purpose processors. Adaptive antenna arrays, or simply, adaptive arrays, periodically analyze the signals received from each of the antennas in an array to distinguish between desired signals (e.g., from a desired remote user terminal, such as cellular telephone or other communication device) and undesired signals (e.g., uplink signals of undesired remote user terminals in the same or different cell area), multipath, etc. Other types of antenna array systems, and in particular, switched beam antenna array systems, also may be employed, although such types of antenna array systems typically do not dynamically and adaptively vary their radiation pattern to mitigate interference, but are limited to a finite number of beamforming patterns.
The process of combining the signals of a number of antenna elements to enhance the gain at the location of a desired remote user terminal, while diminishing gain at the location of one or more other remote user terminals, is generally referred to as beamforming. A downlink weight is computed by the antenna array system for describing a downlink beamforming strategy that provides a suitable radiation pattern for transmission of signals from the antenna array system to a desired remote user terminal. Conversely, an uplink weight is determined by the antenna array system for describing an uplink beamforming strategy that provides a suitable radiation pattern for reception of signals by the antenna array system.
The weights are generally computed as a function of the spatial and/or temporal characteristics associated one or more remote user terminals, as may be determined, for example, by measurement of uplink signals received at the various antenna elements of the antenna array. For example, in some adaptive array systems, the direction-of-arrival (DOA) measurement performed by an adaptive array system may provide a spatial characteristic associated with an uplink signal, and thus, the source (i.e., the transmitter) of such uplink signal. However, other known spatial characteristics and methods for determining the same exist. As such, it should be appreciated that the description herein does not depend on, and as such, is not limited to, a particular type of spatial characteristic or spatial characteristic measurement technique.
FIG. 1 is a diagram depicting a simplified radiation pattern of one type of antenna array system, according to the prior art. In the system shown in FIG. 1, an antenna array 10 transmits (downlink) signals to and/or receives (uplink) signals from a desired (sometimes referred to as xe2x80x9cprimaryxe2x80x9d) remote user terminal (RUT) 12, such as a mobile or stationary remote user terminal (e.g., a cellular voice and/or data communication device, a PDA having wireless capability, a modem or other wireless communication interface coupled to a mobile or stationary data processing device, etc.) on one or more xe2x80x9cprimaryxe2x80x9d channels. In accordance with known xe2x80x9csmart antennaxe2x80x9d or xe2x80x9cadaptive arrayxe2x80x9d processing techniques, the antenna array 10 may, depending on a number of factors, also simultaneously generate regions of interference mitigation (or xe2x80x9cnullsxe2x80x9d) toward other RUTs. As such, in FIG. 1, the antenna array 10 generates an enhanced gain region 6 at the location of the desired RUT 12, while also generating a first region of relatively minimized gain or xe2x80x9cnullxe2x80x9d region 2 at the location of an undesired RUT 14 and a second interference mitigated or xe2x80x9cnullxe2x80x9d region 4 at the location of another undesired RUT 16.
The null regions 2 and 4 represent one of the advantages of adaptive arrays and xe2x80x9csmart antennaxe2x80x9d processing. In particular, each of the nulls 2 and 4 represent a represent a region of minimized gain, relative to the enhanced gain region 6. As such, the antenna array 10 typically, when transmitting to the desired RUT 12 on a primary channel also generates a null at one or more locations, where each location generally corresponds to the location of another RUT. By so doing, the antenna array 10 may mitigate the interference that one or more other RUTs experience when the antenna array 10 communicates with the desired RUT 12. As such, null generation may be viewed as a technique for providing interference mitigation, and each xe2x80x9cnull regionxe2x80x9d may be referred to as an interference mitigated region.
By enhancing the gain at the location of a desired remote user terminal, while diminishing the gain at the location of one or more other remote user terminals, the antenna array 10 may xe2x80x9cspatiallyxe2x80x9d receive and transmit signals, and as such, increase system capacity, decrease interference experienced by or caused by other remote user terminals, etc., by focusing transmission and/or reception gain at the location of a desired RUT, while diminishing transmission and/or reception gain at the location of one or more undesired RUTs.
It should be appreciated that the term xe2x80x9cnullxe2x80x9d as used in the context of adaptive array systems does not typically mean a region of zero electromagnetic energy, since nulls may often include some level of gain, though typically less than an enhanced region. Furthermore, depending on various factors, including the power delivery constraints for the desired RUT, an adaptive array system may vary the xe2x80x9camountxe2x80x9d of nulling by varying the number of nulls generated and/or varying the intensity/depth of nulls, such that the closer a null is to zero gain, the more intense or deep the null.
Unfortunately, adaptive arrays typically direct interference mitigation (or xe2x80x9cnullingxe2x80x9d) toward an RUT occupying the same primary channel (e.g., time slot or carrier frequency slot) as a desired RUT. As such, the above-mentioned effects of inter-channel interference typically exist, even in adaptive array systems.
Thus, what is desired is a method and system for reducing inter-channel interference in a wireless system.